During the operation of direct drive fuel injectors in particular, which have a magnetic armature, which is mechanically coupled to a valve needle, and a coil drive having a coil for moving the magnetic armature, with identical current/voltage parameters, because of electrical, magnetic, and/or mechanical tolerances, differing opening and/or closing behavior over time of the individual fuel injectors occurs. This in turn results in undesired injector-individual variations in the quantity of the actually injected fuel.
The relative injection quantity differences from fuel injector to fuel injector are increased, however, as injection times become shorter and therefore injection quantities become smaller. It is already important for modern engines, and in consideration of a further reduction of pollutant emissions it will become still more important for future engine generations, that a high quantity precision can also be ensured in the case of small fuel quantities to be injected. A high quantity precision can only be achieved, however, if the actual movement behavior of the valve needle or of the magnetic armature is known in particular during the opening operation and/or during the closing operation.
The coil current required for operating a fuel injector having a coil drive is typically provided by suitable current regulator hardware. The resulting chronological progression of the current through the coil of the coil drive is dependent in this case, inter alia, on the inductance and the electrical resistance of the coil. The electrical resistance is composed of the ohmic resistance of the turn(s) of the coil and the resistance of the (ferro-)magnetic material of the fuel injector. Eddy currents, which flow in the ferromagnetic material because of magnetic flux changes, are damped by the finite electrical resistance of the (ferro-)magnetic material.
The end of an opening movement of the magnetic armature or the valve needle (the magnetic armature stops on a mechanical opening stop) and also the end of the following closing movement of the magnetic armature or the valve needle (the magnetic armature stops on a valve seat) can be determined by means of a precise evaluation of the exact chronological progression of the coil current or the coil voltage. Specifically, these ends are each recognizable as a bend in the progression of the coil current or the coil voltage.